Drainback Solar Water Heater

ABSTRACT

A solar water heater includes a drainback unit with a drainback reservoir and an anti-airlock conduit for assuring that working fluid in the solar collectors is consistently drained from the solar collectors into a drainback reservoir once circulation the working fluid in the solar collectors has stopped. The drainback unit also provides rapid startup by positioning the heat exchanger outside of the drainback reservoir and by positioning the inlet and outlet of drainback reservoir in close proximity to each other.

CLAIM OF PRIORITY

The present patent application claims priority from U.S. Provisional Application No. 61/254,380, filed Oct. 23, 2009 and U.S. Provisional Application No. 61/258,262, filed Nov. 5, 2009, both of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a solar water heater with a drainback unit to drain fluid from solar heat collectors connected to the solar water heater.

BACKGROUND OF THE INVENTION

A conventional solar water heater includes one or more solar collectors for heating a working fluid, a storage tank for storing heated water, and a heat exchanger for transferring the heat in the working fluid to the water in the storage tank. Particularly, the conventional solar water heater has a solar heat exchanger circuit comprising the solar collectors, a solar hot fluid conduit, a solar heat exchanger coil in the heat exchanger, a solar cold fluid conduit, and a solar pump for circulating the working fluid through cold fluid line, through the solar collectors, through the hot fluid conduit, and through the solar heat exchanger coil when heat is being captured by the solar collectors. In addition, the conventional solar water heater has a storage tank heat exchanger circuit comprising the storage tank, cold water siphon conduit, a storage tank heat exchanger coil in the heat exchanger, a hot water return conduit, and a hot water pump for circulating water in the storage tank through the storage tank heat exchanger coil.

During operation of such a conventional solar water heater, circumstances arise when the working fluid in the solar collectors should be drained from the solar collectors. In order to extend the life of the solar collectors, the working fluid is typically drained from the solar collectors any time that the solar pump is shut down, and the working fluid is not circulating through the solar collectors. The solar pump is shutdown when the requirement for heat for heating the water in the storage tank has ended or when heat is not available from the solar collectors because of the absence of sunlight. By draining the solar collectors, the solar collectors are protected from freezing and corrosion and the working fluid is protected from degradation.

Draining the working fluid from the solar collectors requires a drainback unit that is part of the solar heat exchanger circuit and that includes a drainback reservoir for storing the working fluid and the necessary plumbing to allow the working fluid to drain from the solar collectors into the drainback reservoir. Conventionally, the heat exchanger is located within the drainback reservoir. When the conventional solar water heater frequently cycles between a heating operation with the solar pump running and a drain back operation with the solar pump shut off, the working fluid in the drainback reservoir typically remains at high temperature, and heat from the working fluid in the drainback reservoir is transferred to the heat exchanger located within the drainback reservoir. When the cycle time between the heating operation with a solar pump running and the drain back operation is long, such as overnight, the working fluid in the drainback reservoir cools, and the working fluid in the heat exchanger and the water in the heat exchanger both become cool. On restart, when heat is again available from the solar collectors, the solar collectors must heat all of the working fluid in the solar heat exchange circuit including all of the drainback working fluid before heat can be transferred by the heat exchanger to the water circulating in the heat exchanger. Consequently, such a conventional drainback unit experiences a substantial startup delay for the delivery of heat to the water in the water storage tank.

In order for the working fluid to drain into the drainback reservoir, air must displace the working fluid in the solar collectors as the working fluid is drained. Under certain operating conditions, an airlock may develop in the plumbing between the solar collectors and the drainback reservoir thereby preventing the working fluid in the solar collectors from draining into the drainback reservoir. Under such circumstances, the trapped working fluid may freeze in the solar collectors thereby damaging the solar collectors, the trapped working fluid may corrode the solar collectors, or the heat transfer characteristics of the trapped working fluid may, when subjected to extreme cold, degrade.

SUMMARY OF THE INVENTION

The drainback unit of the present invention addresses both the problem of startup delay described above and the problem of an airlock inhibiting the drain back of working fluid into the drainback reservoir once the solar pump has stopped circulating the working fluid in the solar collectors of the solar heat exchanger circuit.

In order in order to assure that the working fluid in the solar collectors is consistently drained from the solar collectors, the drainback unit of the present invention has an anti-airlock conduit connecting air in the top of the drainback reservoir to the solar hot fluid conduit to allow the air in the drainback reservoir to bubble through the solar hot fluid conduit to the solar collectors while the working fluid drains by the force of gravity through the solar cold fluid conduit and through the solar pump into the bottom of the drainback reservoir.

In order to speed the delivery of heat to the water in the storage tank up on restart of the solar pump, the heat exchanger of the drainback unit in accordance with the present invention is located outside of the drainback reservoir. Moreover, the drainback reservoir outlet conduit and the drainback reservoir inlet conduit are connected to the drainback reservoir near the bottom of the drainback reservoir and in close proximity to each other. Consequently, upon restart, the solar collectors need only heat the working fluid in the solar collectors, the solar hot fluid conduit, the solar cold fluid conduit a small layer of working fluid near the bottom of the drainback reservoir and before the heat exchanger can begin delivering heat to the water in the storage tank.

Further objects, features and advantages will become apparent upon consideration of the following detailed description of the invention when taken in conjunction with the drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a solar water heater with a drainback unit in accordance with the present invention.

FIG. 2 is a perspective view of the drainback unit for the solar water heater in accordance with the present invention.

FIG. 3 is a perspective elevation view of the drainback unit (cover removed) for the solar water heater in accordance with the present invention.

FIG. 4 is a detailed section view of the anti-airlock conduit connecting the solar hot fluid conduit to the drainback reservoir of the drainback unit in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning to FIG. 1, a solar water heater 10 in accordance with the present invention comprises a storage tank 16 for holding heated water, one or more solar collectors 12 charged with a working fluid, a heat exchanger 14 for transferring heat, during a heating operation, from the working fluid to the water in the storage tank, and a drainback unit 40 (FIGS. 2 and 3) for collecting the working fluid when the heating operation has terminated. The storage tank 16 forms part of a storage tank heat exchanger circuit 22, which comprises the storage tank 16, a cold water siphon conduit 32, a heat exchanger cold water inlet 28, a storage tank heat exchanger coil 24 inside the heat exchanger 14, a heat exchanger hot water outlet 30, a storage tank water pump 26, a hot water return conduit 34, and an air vent 27, all connected in series as shown in FIG. 1.

The solar collectors 12 form part of a solar heat exchange circuit 46, which comprises the solar collectors 12, a solar hot fluid conduit 54, a solar heat exchanger coil 48 inside the heat exchanger 14, a drainback reservoir inlet conduit 62, a drainback reservoir 44, a drainback reservoir outlet conduit 60, a solar fluid pump 50, and a solar cold fluid conduit 52, all connected in series as shown in FIG. 1. The drainback reservoir 44 and an anti-airlock conduit 56 connected between the solar hot fluid conduit 54 and the top of the drainback reservoir 44 comprise the drainback unit 40. The anti-airlock conduit 56 of the drainback unit 40 is illustrated in greater detail in FIG. 4. As shown in FIG. 3, the heat exchanger 44 is physically located outside of the drainback reservoir 44. Further, the drainback reservoir outlet conduit 60 and the drainback reservoir inlet conduit 62 are located adjacent to each other and near the bottom of the drainback reservoir 44.

With continued reference FIG. 1, the storage tank 16 receives cold water from a pressurized source of cold water (not shown) via cold water supply line 21 and storage tank inlet conduit 18. Hot water in the storage tank 16 is delivered from the storage tank 16 to a water system (not shown) via a storage tank outlet conduit 20, a mixing valve 19, and a hot water output line 23. In order to store the most amount of heat in the storage tank 16, the water in the storage tank 16 is maintained at a temperature well above the temperature of the water required by the water system. The mixing valve 19 serves to reduce the temperature of the water flowing in the storage tank outlet conduit 20 by injecting cold water from the cold water supply line 21 to produce water in the hot water output line 23 that is of the appropriate temperature for use by the water system. The storage tank 16 is further equipped with a conventional temperature/pressure relief valve 25 to prevent over temperature or over pressure build up in the storage tank 16.

In order to heat the water in the storage tank 16, solar energy is collected by the solar collectors 12 which in turn heat the working fluid in the solar heat exchanger circuit 46. The working fluid may include among other fluids, water, glycol, glycol/water mixtures, alcohols, alcohol/water mixtures, and other heat transfer fluids known to those persons of ordinary skill in the art. During a water heating operation, the working fluid in solar heat exchanger circuit 46 is circulated by means of solar fluid pump 54 in the direction shown by the arrows in FIG. 1. Cool working fluid is drawn from the drainback reservoir 44 through the drainback reservoir outlet conduit 60 by the solar fluid pump 54 and forced through solar cold fluid conduit 52 into the solar collectors 12. As the working fluid passes through the solar collectors 12, the working fluid is heated and exits through solar hot fluid conduit 54. A hot fluid temperature sensor 72 is connected to the solar hot fluid conduit 54 adjacent the solar collectors 12 to determine the temperature of the working fluid as it exits the solar collectors 12. The working fluid then passes from the solar hot fluid conduit 54 into the solar heat exchanger coil 48 of the heat exchanger 14. In the heat exchanger 14, the working fluid gives up its heat to the storage tank heat exchanger coil 24, and the working fluid, in a cooler state, exits the solar heat exchanger coil 48 and into the drainback reservoir 44 through drainback reservoir inlet conduit 62. The working fluid in the drainback reservoir 44 is withdrawn through drainback reservoir outlet conduit 60 by the solar fluid pump 50 and the heating operation cycle continues. The drainback reservoir 44 is equipped with a pressure relief valve 45 to accommodate any overpressure condition that might exist inside the drainback reservoir 44.

The water in the storage tank heat exchanger circuit 22 as previously stated is heated in the heat exchanger 14 when the working fluid in the solar heat exchanger coil 48 gives up its heat to the storage tank heat exchanger coil 24. The water in the storage tank heat exchanger circuit 22 is circulated by means of the storage tank water pump 26. The storage tank water pump 26 circulates the water in the storage tank heat exchanger circuit 22 by drawing the water in the storage tank 16 through the cold water siphon conduit 32, the heat exchanger cold inlet 28, the storage tank heat exchanger coil 24 of the heat exchanger 14, and the heat exchanger hot outlet 30. The storage tank water pump 26 then forces the heated water through the hot water return conduit 24 and back into the storage tank 16. An air vent valve 27 is provided on the hot water return conduit 24 adjacent the storage tank 16 to allow for the exhaustion and intake of air to and from the storage tank 16 as the water level in the storage tank 16 rises and falls. A storage tank temperature sensor 70 is connected to the storage tank 16 to sense the temperature of the water in the storage tank 16.

The operations of the storage tank water pump 26 and the solar fluid pump 50 are controlled by the control module 74. The control module 74 monitors the temperature of the working fluid in the solar hot fluid conduit 54 as sensed by the hot fluid temperature sensor 72 and the temperature of the water in the storage tank 16 as sensed by the storage tank temperature sensor 70. When the temperature of the working fluid exceeds the temperature of the water in the storage tank, typically by a differential of 16° F., the control module 74 recognizes that the solar collectors 12 are producing sufficient heat to begin heating the water in the storage tank 16. At that point, both the storage tank water pump 26 and the solar fluid pump 50 are turned on, and a heating operation is commenced. Once the differential between the hotter working fluid in the solar collectors 12 and the water in the storage tank 16 drops to a predetermined differential value, typically 6° F., the control module 74 recognizes that the water temperature is sufficiently high, the storage tank water pump 26 and the solar fluid pump 50 are shut off, and the heating operation ceases. The control module 74 also monitors both the working fluid temperature and the water temperature, by means of the hot fluid temperature sensor 72 and in the storage tank temperature sensor 70, for safe maximum temperatures.

Once a heating operation has ceased, the working fluid should be drained from the solar collectors 12 and stored in the drainback reservoir 44. As the working fluid drains from the solar collectors 12 through the solar cold fluid conduit 52 and the solar fluid pump 50 and into the drainback reservoir 44, air must flow into the solar collectors 12 through the solar hot fluid conduit 54, or the solar collectors 12 become airlocked, and the working fluid will not drain from the solar collectors 12. In order to supply vacuum relief air to the solar collectors 12 through the solar hot fluid conduit 54, the present invention includes an anti-airlock conduit 56 connected between the top of the drainback reservoir 44, where there is a supply of air, and the solar hot fluid conduit 54 as shown in detail in FIG. 4. The anti-airlock conduit 56 has a cross sectional area that is less than half the size of the cross-sectional area of the solar hot fluid conduit 54. Through testing, the preferred ratio of the cross-sectional area of the anti-airlock conduit 56 to the cross-sectional area of the solar hot fluid conduit 54 is approximately 1:36, which is the ratio resulting from the use of a ¾ inch solar hot fluid conduit 54 and a ⅛ inch anti-airlock conduit 56. The proper ratio of the cross-sectional areas ensures that sufficient air can bubble up through the solar hot fluid conduit 54 during a drain back operation to displace the working fluid that drains from the solar collectors 12 through the solar cold fluid conduit 52. By the same token, the proper ratio of the cross-sectional areas ensures that during a heating operation, significant amounts of working fluid circulating through the solar hot fluid conduit 54 are not diverted into the drainback reservoir 44 instead of into the heat exchanger 14.

By locating the heat exchanger 14 outside of the drainback reservoir 44 and by placing the drainback reservoir inlet conduit 62 from the solar heat exchanger coil 48 adjacent the drainback reservoir outlet conduit 60, the delay in providing heat at the startup of a heating operation after a drain back operation can be minimized. Particularly, where the working fluid has been drained from the solar collector 12 overnight or for an extended period of time, the temperature of the working fluid in the drainback reservoir 44 will be cold. Consequently, the amount of time required for startup depends on how quickly the circulating working fluid can be heated in the solar collectors 12. The startup time depends on the amount of working fluid that must be heated by the solar collectors 12. By limiting the amount of working fluid to only the working fluid in the solar collectors 12, in the solar cold fluid conduit 52, in the solar hot fluid conduit 54 and in small layer of working fluid in the drainback reservoir 44 adjacent the drainback reservoir outlet conduit 60 and the drainback reservoir inlet conduit 62, the startup delay can be minimized

While this invention has been described with reference to preferred embodiments thereof, it is to be understood that variations and modifications can be affected within the spirit and scope of the invention as described herein and as described in the appended claims. 

1. Solar water heater comprising: a. a water storage tank heat exchanger circuit comprising: i. a water storage tank; ii. a cold water siphon conduit; iii. a hot water return conduit; iv. a storage tank heat exchanger coil in a heat exchanger; and v. storage tank water pump, wherein the water storage tank is connected to the storage tank heat exchanger coil by means of the cold water siphon conduit and the hot water return conduit and wherein the storage tank water pump circulates water through the water storage tank and through the storage tank heat exchanger coil; b. a solar heat exchanger circuit comprising: i. a solar collector; ii. a solar heat exchanger coil in the heat exchanger; iii. a cold fluid conduit; iv. a hot fluid conduit having a cross-sectional area; v. a solar fluid pump; and vi. a drainback unit including (a) a drainback reservoir; and (b) an anti-airlock conduit having a cross-sectional area for supplying air to the solar collector during a drainback operation, wherein the solar collector is connected to the solar heat exchanger coil by means of the solar hot fluid conduit, the solar cold fluid conduit, and the drainback reservoir and wherein the solar fluid pump circulates working fluid through the solar collector, the solar heat exchanger coil, and the drainback reservoir; c. a control module for turning on and shutting off the storage tank water pump and the solar fluid pump, wherein the control module turns on the storage tank water pump and the solar fluid pump during a heating operation and shuts off the storage tank water pump and the solar fluid pump during a drain back operation.
 2. The solar water heater of claim 1, wherein the anti-airlock conduit is connected adjacent the top of the drainback reservoir and the solar hot fluid conduit.
 3. The solar water heater of claim 2, wherein the cross-sectional area of the anti-airlock conduit is less than half of the cross-sectional area of the hot fluid conduit.
 4. The solar water heater of claim 3, wherein a ratio between the cross-sectional area of the anti-airlock conduit and the cross-sectional area of the hot fluid conduit is approximately 1:36.
 5. The solar water heater of claim 1, wherein solar water heater includes a storage tank temperature sensor for sensing the temperature of the water in the water storage tank and a hot fluid temperature sensor for sensing the temperature of the working fluid exiting the solar collector.
 6. The solar water heater of claim 5, wherein the control module turns on the storage tank water pump and the solar fluid pump, to initiate a heating operation, when the temperature sensed by the hot fluid temperature sensor exceeds the temperature sensed by the storage tank temperature sensor by a first predetermined value.
 7. The solar water heater of claim 5, wherein the control module shuts off the storage tank water pump and the solar fluid pump, to initiate a drain back operation, when the temperature sensed by the hot fluid temperature sensor exceeds the temperature sensed by the storage tank temperature sensor by a second predetermined value.
 8. The solar water heater of claim 4, wherein the control module shuts off the storage tank water pump and the solar fluid pump when the temperature sensed by the storage tank temperature sensor reaches a third predetermined maximum value.
 9. The solar water heater of claim 1, wherein the heat exchanger is positioned outside of drainback reservoir.
 10. The solar water heater of claim 9, wherein the drainback reservoir includes a drainback reservoir inlet conduit for receiving working fluid from the solar heat exchanger coil and a drainback reservoir outlet conduit for supplying working fluid to the solar cold fluid conduit and wherein the drainback reservoir inlet conduit and the drainback reservoir outlet conduit are connected to the drainback reservoir in close proximity to each other.
 11. Solar water heater comprising: a. a water storage tank heat exchanger circuit comprising: i. a water storage tank; ii. a cold water siphon conduit; iii. a hot water return conduit; iv. a storage tank heat exchanger coil in a heat exchanger; and v. storage tank water pump, wherein the water storage tank is connected to the storage tank heat exchanger coil by means of the cold water siphon conduit and the hot water return conduit and wherein the storage tank water pump circulates water through the water storage tank and through the storage tank heat exchanger coil; b. a solar heat exchanger circuit comprising: i. a solar collector; ii. a solar heat exchanger coil in the heat exchanger; iii. a cold fluid conduit; iv. a hot fluid conduit; v. a solar fluid pump; and (a) a drainback unit including a drainback reservoir, wherein the solar collector is connected to the solar heat exchanger coil by means of the solar hot fluid conduit, the solar cold fluid conduit, and the drainback reservoir, wherein the heat exchanger is positioned outside of the drainback reservoir, and wherein the solar fluid pump circulates working fluid through the solar collector, the solar heat exchanger coil, and the drainback reservoir; c. a control module for turning on and shutting off the storage tank water pump and the solar fluid pump, wherein the control module turns on the storage tank water pump and the solar fluid pump during a heating operation and shuts off the storage tank water pump and the solar fluid pump during a drain back operation.
 12. The solar water heater of claim 11, wherein the drainback reservoir includes a drainback reservoir inlet conduit for receiving working fluid from the solar heat exchanger coil and a drainback reservoir outlet conduit for supplying working fluid to the solar cold fluid conduit and wherein the drainback reservoir inlet conduit and the drainback reservoir outlet conduit are connected to the drainback reservoir in close proximity to each other. 